The lithium cells have been intensively developed during the recent two decades enabling thus existence of many portable devices. Nevertheless the growing demands for higher capacity and safety of lithium batteries do not always comply. This slows down progress of many applications, including the substitution of lead-acid accumulators with lithium accumulators possessing higher voltage, or development of large batteries for electro mobiles and energy storage.
The prior art technologies using graphite as an active material for the negative electrode are not able to ensure safety of a battery with the weight exceeding 0.5-1 kg. The efforts to increase the size of this type of accumulators encounter many problems such as overheating, intermediate layer on the graphite, swelling, development of metal lithium on the graphite surface and a risk of explosion or fire. These safety problems push the large lithium accumulators beyond the limits of acceptability.
Technologies substituting graphite with a different material, e.g. lithium titanate spinel Li4Ti5O12 (LTS) strongly improve the safety parameters of lithium batteries, but on the other hand, they significantly decrease the cell voltage.
The lithium batteries manufactured on this basis meet the safety demands for use in electro mobiles, but the weight parameters of such batteries don't allow their easy use in small vehicles.
All rechargeable lithium accumulators manufactured today are based on planar electrodes, where a mixture of an active material, conductive carbon and organic binding agent are laminated in a thin layer onto a conductive foil, usually aluminum or copper (current collector). The thickness of these planar electrodes usually does not exceed 200 micrometers. The positive and negative electrodes are stacked together separated by a thin layer of an electrically insulating material, usually a perforated foil made of an organic polymer—separator. The stacked thin-film electrodes insulated by the separators are then pressed together, placed into the accumulator package and the space inside the accumulator is filled with an electrolyte. A non-aqueous solution of lithium salts is used as an electrolyte. In connection with such devices based on the planar electrodes, it is most important to prevent the growth of lithium metal during the charging and discharging process e.g. when the charging or discharging is too fast. The lithium metal develops on electrodes in the form of dendrites, which overgrow through the separator and cause an electric shortage between both electrodes. Any use of metal lithium as a negative electrode in the planar thin-film configuration accumulator is impossible for the same reason.
One type of a cell with thin-film planar electrodes is described in detail in U.S. Pat. No. 6,197,450. Despite its increased volumetric capacity, this type is affected by inherent properties of planar electrodes as described above.
One of possible compositions of a lithium battery with a thin-film planar electrode configuration is described in US. pat. application 2007/0092798. Active nano-materials are used as a component of the electrodes. The battery cell arranged in planar configuration shows a relative low volumetric capacity, which is further limited by the type of cathode material.
Another US pat. application 2007/0134554 teaches a carbon electron conductor deposited on solid particles of a specific active material. The carbon improving the conductivity of the active material is to be formed directly on the surface of the active material using a rather complicated process of pyrolysis.
EP1244168A discloses the formation of thin layers of an electrochemical cell by coating a suitable substrate with a paste comprising the active material, organic binders and conductive carbon without application of a sintering process. The calculation of a model example 8, where a separator of 50-90% porosity is used, shows a gradient of the electrode's voltage with the electrical potential sharply dropping down with the increasing thickness of the electrode. Based on this fact, it is to be understood that the disclosed network can not be used for the formation of electrodes of higher thickness, for example exceeding 0.5 mm.